Ripple effect explains Phantom traffic jams

Motorists spend six months of their lives in traffic jams – many of them phantom bottlenecks with no apparent explanation, scientists have revealed.

In a study funded by the Engineering and Physical Sciences Research Council (EPSRC), Eddie Wilson of Bristol University believes he has identified the reasons behind the frustrating phenomena.

If a motorist gets too close to the car in front and hits the brakes, the driver behind is forced to do the same.

This can force hundreds of other motorists to brake and within moments create a ripple effect that can stretch for miles.

“The stop-and-go waves are generated by very small events at the level of individual vehicles,” said Dr Wilson.

“It can just be bad lane changing, which increases to such an extent that the impact is much bigger than the original event which caused it.

“In certain situations a tipping point is reached that magnifies small effects to create large changes that can involve hundreds of vehicles and which may be a couple of miles long.

“We’ve seen individual phantom jams that have travelled over 50 miles down the motorway — and on Bank Holiday Fridays, the entire M6 from Birmingham to the Lake District is often stop-go the whole way,” he added.

Dr Wilson tracked the movement of cars along a 10-mile stretch of the M42 near Birmingham, then created a computer model to help explain why traffic suddenly built up and then dissipated just as quickly.

Andrew Howard, head of road safety at the AA, was not surprised by the findings: “The first driver brakes and the one behind has to brake a little bit harder and so on, until somebody comes to a complete halt,” he said.

“This is why we are getting variable speed limits now. There is a real benefit if you can get people going at a constant 50mph rather than stopping and starting at 70mph.

“We are finding that if you slow cars down you get better fuel economy and faster journey times.”